U.S. patent number 6,646,418 [Application Number 10/202,594] was granted by the patent office on 2003-11-11 for method and apparatus for fuel cell protection.
This patent grant is currently assigned to Motorola, Inc.. Invention is credited to Jerald A. Hallmark, Chenggang Xie.
United States Patent |
6,646,418 |
Xie , et al. |
November 11, 2003 |
Method and apparatus for fuel cell protection
Abstract
A fuel cell system is protected by monitoring at least one fuel
cell parameter, comparing the parameter to a preset level, and
disconnecting or reconnecting a main load in response to the fuel
cell parameter. For example, a fuel cell system (300) is provided
with a protection circuit (304, 308) that prevents operation of the
fuel cells in the negative dP/dI region. System (300) includes a
stack of fuel cells (302) connected in series and coupled to a main
load (310). A controller (304) provides a control signal (314)
based on the individual fuel cell voltage levels falling above or
below a preset level. Control signal (314)is used to control a load
switch (308)coupled between the stack of fuel cells (302) and the
main load (310). The load switch (308) disconnects the main load
(310) in order to prevent operation of the fuel cell cells in the
negative dP/dI region.
Inventors: |
Xie; Chenggang (Phoenix,
AZ), Hallmark; Jerald A. (Gilbert, AZ) |
Assignee: |
Motorola, Inc. (Schaumburg,
IL)
|
Family
ID: |
29400920 |
Appl.
No.: |
10/202,594 |
Filed: |
July 24, 2002 |
Current U.S.
Class: |
320/120;
320/121 |
Current CPC
Class: |
H01M
16/006 (20130101); H01M 10/46 (20130101); G01R
31/396 (20190101); H01M 8/04589 (20130101); H01M
8/04947 (20130101); H01M 8/04559 (20130101); H01M
8/04582 (20130101); H01M 8/04552 (20130101); Y02E
60/50 (20130101); Y02E 60/10 (20130101); G01R
19/16542 (20130101) |
Current International
Class: |
G01R
31/36 (20060101); H01M 16/00 (20060101); H01M
10/46 (20060101); H01M 8/00 (20060101); H01M
10/42 (20060101); H02J 7/34 (20060101); H01M
8/04 (20060101); H01M 010/46 () |
Field of
Search: |
;320/103,116,118,119,120,121,122,161,162,163,164 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Tso; Edward H.
Attorney, Agent or Firm: Doutre; Barbara R. Gilmore; Douglas
W.
Claims
We claim:
1. A fuel cell system, comprising: a plurality of fuel cells
connected in series; a voltage monitor for monitoring an individual
voltage of each cell; a precision voltage reference to which each
individual cell voltage is compared to a preset level; a load
operatively coupled to the plurality of fuel cells; and a load
switch coupled between the plurality of fuel cells and the load,
the load switch disconnecting the load from the system when the
voltage of any cell falls below the preset level and for
reconnecting the load when the cell voltage rises above the preset
level.
2. The fuel cell system of claim 1, wherein the load switch is kept
off for a predetermined recovery time after the cell voltage rises
above the preset level.
3. The fuel cell system of claim 1, further including a timing
circuit to control the load switch.
4. The fuel cell system of claim 1, wherein the fuel cell system is
used as a charger for a battery.
5. A method of protecting a fuel cell system, comprising: providing
a stack of fuel cells connected in series and coupled to a load;
monitoring a voltage of each fuel cell; comparing each monitored
voltage to a preset level; switching the load off in response to
the voltage falling below the preset level; and switching the load
on in response to the voltage rising above the preset level.
6. The method of claim 5, wherein switching the load off further
includes maintaining the load switched off for a predetermined
amount of time after the voltage rises above the preset level.
7. A fuel cell system, comprising: a stack of fuel cells connected
in series; a controller coupled to the stack, the controller
providing individual cell voltage monitoring capability, a
reference voltage, and comparator functionality to generate a
control signal indicating that one or more individual cell voltages
has fallen above or below a preset level; a DC/DC converter for
sourcing power from the stack and providing a converted output
voltage; a load switch for receiving the control signal and the
converted output voltage; and a main load coupled to the load
switch, the load switch responsive to the control signal for
switching in and out the main load.
8. The fuel cell system of claim 7, further comprising a secondary
load providing fuel cell support circuitry coupled to the DC/DC
converter.
9. A fuel cell system, comprising: a plurality of fuel cells
connected in series and coupled to a load; a controller for
determining individual fuel cell voltage level and providing a
control signal based on the individual fuel cell voltage levels,
the control signal indicating whether fuel cell operation is in or
out of negative dP/dI region; and a load switch coupled between the
plurality of fuel cells and the main load, the load switch
disconnecting the load from the fuel cells in response to the
control signal to prevent operation of the fuel cell system in the
negative dP/dI region.
10. A fuel cell system, including: a stack of fuel cells coupled in
series; a controller coupled to the stack; a load operatively
coupled to the fuel cells; and the controller monitoring each fuel
cell voltage, comparing each fuel cell voltage to a reference
voltage, and generating a control signal for disconnecting and
reconnecting the main load.
11. The fuel cell system of claim 10, further comprising a first
DC/DC converter powered by the stack of fuel cells and turned on or
off by the control signal, the DC/DC converter for powering the
main load when turned on by the control signal.
12. The fuel cell system of claim 11, further comprising: a second
DC/DC converter powered by the stack of fuel cells; a second load
comprising fuel cell support circuitry coupled to the fuel cells;
and wherein the second DC/DC converter generates a converted
voltage output for powering the fuel cell support circuitry.
13. The fuel cell system of claim 10, further comprising: a DC/DC
converter powered by the stack of fuel cells and generating a
converted output voltage; and a load switch for receiving the
control signal from the controller and for receiving the converted
output voltage from the DC/DC converter, the load switch for
connecting and disconnecting the main load to the converted output
voltage in response to the control signal.
14. A method of protecting a fuel cell system, comprising:
providing a stack of fuel cells connected in series and coupled to
a load; monitoring the voltage of more than one cell within the
stack; comparing the monitored voltage to a preset level; switching
the load off in response to the voltage falling below the preset
level; and switching the load on in response to the voltage rising
above the preset level.
15. The method of claim 14, wherein the step of monitoring the
voltage of more than one cell comprises monitoring the voltage of
the stack.
16. The method of claim 14, wherein the step of monitoring the
voltage of more than one cell comprises monitoring the voltage of a
group of cells within the stack.
17. A method of protecting a fuel cell system, comprising:
providing a stack of fuel cells connected in series and coupled to
a load; monitoring the voltage of the stack; tracking power drawn
by the load; determining slope of the power versus voltage;
switching the load off if the slope is zero or positive; and
switching the load on in response to the slope being negative.
18. The method of claim 17, wherein the step of tracking power
comprises measuring total cell current and total cell voltage and
calculating the product as the power.
19. A method of protecting a fuel cell system, comprising:
providing a stack of fuel cells connected in series and coupled to
a load; monitoring the current of the stack; tracking power drawn
by the load; determining a slope of the power versus current;
switching the load off, in response to the slope being zero or
negative; and switching the load on, in response to the slope being
positive.
20. The method of claim 19, wherein the step of tracking the power
comprises measuring total cell current and total cell voltage and
calculating the product as the power.
21. A method of protecting a fuel cell system, comprising:
providing a stack of fuel cells coupled in parallel and coupled to
a load; monitoring a current of each fuel cell; comparing each
monitored current to a preset level; switching the load off in
response to the current rising above the preset level; and
switching the load on in response to the current falling below the
preset level.
22. The method of claim 21, wherein switching the load off further
includes maintaining the load switched off for a predetermined
amount of time after the current falls below the preset level.
23. A fuel cell system, including: a stack of fuel cells; a
controller coupled to the stack; a load operatively coupled to the
fuel cells; and the controller monitoring a fuel cell parameter,
comparing the fuel cell parameter to a preset level, and generating
a control signal for disconnecting and reconnecting the load
depending on the fuel cell parameter.
24. A method of protecting a fuel cell system, comprising:
providing a stack of fuel cells with a load operatively coupled
thereto; monitoring at least one parameter of one or more of the
fuel cells; comparing the parameter to a preset parameter level;
and disconnecting or reconnecting the load in response to the fuel
cell parameter.
25. The method of claim 24, wherein the stack of fuel cells are
coupled in parallel and the at least one parameter is current.
26. The method of claim 24, wherein the stack of fuel cells are
coupled in series and the at least one parameter is voltage.
27. The method of claim 24, wherein the stack of fuel cells is
coupled in series and the at least one parameter is current through
the stack.
28. The method of claim 24, wherein the step of monitoring at least
one parameter comprises monitoring current and voltage and the
stack of fuel cells are coupled with a combination of series and
parallel coupling.
29. A method of protecting a fuel cell system, comprising:
providing a stack of fuel cells operatively coupled to a load;
monitoring a parameter of the stack; tracking power drawn by the
load; determining slope of the power versus parameter; switching
the load off on or off in response to the slope having a
predetermined characteristic.
30. The method of claim 29, wherein the parameter is voltage and
the load is switched off if the slope is zero or positive.
31. The method of claim 29, wherein the parameter is current and
the load is switched off if the slope is zero or negative.
32. The method of claim 30, wherein the stack of fuel cells
comprises a plurality of fuel cells coupled in series.
33. The method of claim 31, wherein the stack of fuel cells
comprises a plurality of fuel cells coupled in parallel.
Description
FIELD OF THE INVENTION
This invention relates generally to fuel cells and more
specifically to fuel cell protection means.
BACKGROUND OF THE INVENTION
Fuel cells provide clean, direct current (DC) electricity. Fuel
cells convert reactants, namely fuel and oxidant (air or oxygen),
to generate electric power and reaction products. A typical fuel
cell power source is constructed from a stack of cells coupled in
series as shown in FIG. 1. For a series electrical connection 100,
the same amount of current is drawn from each cell 102. In
practice, each cell 102 has slightly different performance
characteristics.
Fuel cells exhibit a decreasing output voltage as the current
output is increased. This curve is not linear and tends to drop off
faster at higher currents. Consequently, as can be seen in FIG. 2,
the power vs. current characteristic 200 generally has a peak 202
at mid-current levels and then rolls off. Below the "peak power"
point 202, as more current is drawn, the power increases. Above the
"peak power" point 202, as more current is drawn, the power
decreases. The region in which the slope of power vs. current is
negative is referred to as negative dP/dI region 204. Beyond point
202, if the system tries to draw more power from the fuel cell, it
quickly goes to low power (low voltage) and typically the support
circuitry shuts off and the whole system stops working. In some
cases, one of the cells may fail to work properly because of a
temporary problem within the cell, such as a blocked micro-channel
caused by particles or carbon dioxide bubbles. Therefore, it is
desirable to monitor the system operation and avoid this
condition.
Prior art fuel cell systems have included systems that monitor and
compare the fuel cell voltage to a reference fuel cell voltage
(Vfc), activating an alarm when the Vfc is exceeded. However, this
type of system only provides for monitoring and alarm activation.
Other systems have utilized a high power resistor and thermistor in
conjunction with monitoring a voltage, and still other systems have
used optoisolators. Again, these systems focus on monitoring the
cell voltage and lack any constructive means of circuit
protection.
Accordingly, a need exists for a protection apparatus and technique
to prevent any cells from operating in the negative dP/dI
region.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is illustrated by way of example and not
limitation in the accompanying figures, in which like references
indicate similar elements, and in which:
FIG. 1 is a block diagram representation of a typical stack of fuel
cells coupled in series;
FIG. 2 is a graph of power versus current for a typical fuel cell
operating under various load conditions;
FIG. 3 is a block diagram of a fuel cell system including a
protection circuit in accordance with a first embodiment of the
invention;
FIG. 4 is a flow chart illustrating a method for protecting a fuel
cell system in accordance with the first embodiment of FIG. 3;
FIG. 5 is a flow chart illustrating a method for protecting a fuel
cell system in accordance with an alternative embodiment of the
invention;
FIG. 6 is a flow chart illustrating a method for protecting a fuel
cell system in accordance with another alternative embodiment of
the invention;
FIG. 7 is a block diagram of a fuel cell system including a
protection circuit in accordance with another alternative
embodiment of the invention; and
FIG. 8 is a flow chart illustrating a method for protecting a fuel
cell system in accordance with another embodiment of the invention
in which the cells are parallel-coupled.
Skilled artisans will appreciate that elements in the figures are
illustrated for simplicity and clarity and have not necessarily
been drawn to scale.
DETAILED DESCRIPTION OF THE DRAWINGS
In the description to follow below, there is provided an apparatus
and technique of fuel cell protection in which a stack of fuel
cells (either series-connected, parallel-connected, or combination
of both) having a load operatively coupled thereto is monitored for
pre-set conditions such that the load can effectively be
temporarily disconnected from the fuel cells thereby placing the
stack in a protective mode. These pre-set conditions include
monitoring a fuel cell parameter, such as voltage, current, or
power and comparing the parameter to a preset level, and then
disconnecting/reconnecting the load when the preset levels are met.
The preferred set of conditions includes monitoring the slope of
power (through the load) versus current (through the cell) for
operation in the positive slope region. The strategies described
below are applicable to hybrid systems, where the fuel cell is used
as a (battery) charger and where the power can be temporarily
interrupted without impacting the overall device performance.
Referring to FIG. 3, there is shown a block diagram of a fuel cell
system 300 having a protection circuit in accordance with a first
embodiment of the invention. System 300 includes a plurality of
fuel cells 302 connected in series, a controller 304, a DC/DC
converter 306, a load switch 308, a main load 310 and a secondary
load 312. Each fuel cell, depending on load, typically generates a
voltage of about 0.25 to about 0.7 volts. When connected in series,
the stack output voltage is the sum of the voltages generated by
the fuel cells. Depending upon the load, and assuming that all fuel
cells are operating efficiently, the stack operates to about 11.2
volts for a stack of 16 cells. Higher voltages can be achieved by
adding more fuel cells to the stack in series depending on the
application.
The main load 310 generally represents a hybrid system, such as a
battery, capacitor, or other energy storage device. The secondary
load 312 operates as fuel cell support circuitry and includes such
items as pumps, valves, mixer, fan, sensor and controlling
electronics. The support system circuitry is well known in the art
and for the sake of simplicity will not be described further.
For the embodiment of system 300, the load switch 308 is coupled
between the plurality of fuel cells 302 and the main load 310. In
accordance with the first embodiment, the load switch 308 operates
by temporarily disconnecting the main load 310 from the system 300
when the voltage of any cell from the plurality of cells 302 falls
below a preset level. Conversely, the load switch 308 reconnects
the main load 310 when the cell voltage rises above the preset
level.
The controller 304 can be any controller/microprocessor type device
known in the art that provides individual cell voltage monitoring
capability, a precision reference voltage, and comparator
functionality. In accordance with the first embodiment, the
controller provides a control signal 314 indicating that one or
more individual cell voltages has reached a preset level. In some
cases, to simplify the system, the sum of more than one cell
voltage, instead of individual cell voltages, is monitored and
compared to a preset level. The load switch 308 receives the
control signal 314 as well as a converted output voltage 316
generated by the DC/DC converter 306. The load switch 308 is
responsive to the control signal 314 for switching in and out the
main load 310. The load switch 308, coupled between the plurality
of fuel cells 302 and the main load 310, disconnects the main load
from the fuel cell system 300 in response to the control signal 314
to prevent operation of the fuel cell system in the negative dP/dI
region as previously described in FIG. 2. A timing circuit can be
added, if desired, to control switching in and out the main load
310 or sourcing power on or off to the load. To give more time for
the system to recover, it may also be desirable to provide a short
recovery time period after the cell voltage rises above the preset
level.
Referring now to FIG. 4, there is shown a flow chart illustrating a
method 400 for protecting fuel cells from entering the negative
dP/dI region in accordance with the first embodiment of FIG. 3.
Method 400 begins by providing a stack of fuel cells coupled in
series and coupled to a load (step 402). Next, by monitoring the
voltage of each cell (step 404), comparing each voltage to a preset
reference voltage (Vref) (step 406) and disconnecting the load
(step 408) in response to the voltage falling below (or equal to)
the preset level, the system thereby enters a protective mode.
Conversely, the load remains connected at step 410 in response to
the voltage being above the preset level.
By disconnecting the main load, the fuel cell system is prevented
from entering the negative dP/dI region. Method 400 further
includes the steps of continuing to monitor (step 412) each cell
voltage, as the cells are still coupled to the secondary load, and
compare the cell voltages to the preset level (step 414) until each
cell voltage returns above the preset reference level (step 414).
If desired the switch can remain off to allow for additional
recovery time (step 416) before reconnecting the main load (step
418).
As mentioned previously, the sum of more than one cell voltage,
instead of individual cell voltages, can be monitored and compared
to a reference voltage. As for the method 400 described above, step
404 can be replaced with the step of monitoring the voltage of the
stack of cells (or a group of cells within the stack). Step 412 can
be replaced with continuing to monitor and compare the stack
voltage (or group of cells within the stack) until the voltage
returns above the preset reference level.
Another alternative method for protecting fuel cells provides for
tracking the power through the load versus the voltage of the stack
and determining the slope. This method 500 is depicted by the flow
chart of FIG. 5. Method 500 begins with the step of providing a
stack of fuel cells connected in series and coupled to a load (step
502), followed by the steps of monitoring the voltage of the stack
(step 504) and tracking power drawn by the load (step 506), by
measuring total cell current and total cell voltage and taking the
product. Next, the step of determining a slope of the power through
the load versus voltage (step 508) occurs. The system enters a
protection mode by switching the main load off (step 512) in
response to the slope being zero or positive (step 510).
Conversely, the load remains connected (step 514)in response to the
slope being negative. Upon entering the protection mode, the slope
continues to be monitored (step 516), based on the stack being
connected to the secondary load. The main load is reconnected when
the slope reaches an appropriate level (step 518).
Another alternative method for protecting-fuel cells provides for
tracking the power through the load versus the current of the stack
and determining the slope. This method 600 is depicted by the flow
chart of FIG. 6. Method 600 begins with the step of providing a
stack of fuel cells connected in series and coupled to a load (step
602), followed by the steps of monitoring the current of the stack
(step 604) and tracking power drawn by the load (step 606), a
product of cell current and cell voltage. Next, the step of
determining a slope of the power through the load versus current
(step 608) occurs. The system enters a protection mode by switching
the main load off (step 612) in response to the slope being zero or
negative (step 610). Conversely, the load remains connected (step
614) in response to the slope being positive. The slope continues
to be monitored (step 616) based on the stack being coupled to the
secondary load. The main load is reconnected when the slope becomes
positive (step 618).
Another fuel cell protection circuit in accordance with another
alternative embodiment of the invention is shown in FIG. 7. Fuel
cell system 700 includes fuel cells 302, controller 304 with
control signal 314, main load 310, and fuel cell support circuitry
312 as previously described with reference to FIG. 3. Fuel cell
system 700 further includes first and second DC/DC converters 702,
704. The first DC/DC converter 702 is powered by the fuel cells
302. In accordance with this alternative embodiment, the DC/DC
converter 702 is turned on or off based on control signal 314. When
the DC/DC converter 702 is turned on by control signal 314, a
converted voltage output 706 powers the main load 310. When the
DC/DC converter 702 is turned off by control signal 314, the main
load 310 is effectively disconnected. The second DC/DC converter
704 is likewise powered by the fuel cells 302 and produces a
converted output voltage 708 for powering the fuel cell support
circuitry 312 (typically at much lower power). The alternative
embodiment shown in FIG. 7 negates the use of the load switch
described in FIG. 3, but utilizes a second DC/DC converter 702.
Like the fuel cell system 300 of FIG. 3, the controller 304
monitors each cell voltage, compares each cell voltage to the
reference voltage, and generates the control signal 314 for
switching in and out the main load 310 by turning off or on the
DC/DC converter 702.
The technique for protecting fuel cells from entering the negative
dP/dl region described in FIGS. 4, 5, and 6 applies equally well to
the embodiment of FIG. 7. Using the method of FIG. 4 as an example,
the stack of fuel cells is coupled in series and coupled to a main
load (step 402). By monitoring the voltage of each cell (step 404),
comparing each voltage to a preset reference voltage (Vref) (step
406) and disconnecting the main load (step 408) in response to the
voltage falling below (or equal to) the preset level, the system
enters a protective mode. Conversely, the load connection is
maintained (at step 410) if the voltage is above the preset level.
In the alternative embodiment of FIG. 7, the main load 310 is
effectively disconnected by turning off the DC/DC converter 702
with control signal 314.
By disconnecting the main load 310, the fuel cell system 700 is
thereby prevented from entering the negative dP/dI region. As
previously described in reference to FIG. 4, the steps of
continuing to monitor (step 412) and compare the cell voltages to a
preset level (step 414) are performed until each cell voltage
returns above the preset reference level and the load is
reconnected (step 418). If desired, the DC/DC converter 702 can
remain turned off via control signal 314 to allow for additional
recovery time (step 416) before reconnecting the main load (step
418). A timing circuit, preferably included as part of the
controller, can control the recovery time in either system 300 or
700. Although described in terms of the method of FIG. 4, the
alternative methods of FIGS. 5 and 6 are also applicable to system
700.
While described in terms of series-connected fuel cells the fuel
cell protection means of the present invention can be extended to
fuel cells coupled in parallel as well. The apparatus embodiments
of systems 300 and 700 previously described can thus be powered by
parallel-coupled fuel cells. For fuel cell systems in which the
cells are coupled in parallel, current can be monitored instead of
voltage. Thus, the steps can be summarized in FIG. 8, as providing
a stack of fuel cells coupled in parallel and coupled to a load
(step 802), monitoring a current of each fuel cell (or group of
fuel cells)(step 804),comparing each monitored current to a preset
level (step 806), switching the load off (step 808) in response to
the current rising above(or equal to) the preset level and
maintaining the load on (step 810) in response to the current
falling below the preset level. Once the load is switched off (step
808), the current continues to be monitored (step 812) and compared
to a preset reference (step 814). The load is reconnected (step
816) when the current goes below the preset level. Additional
recovery time can be added before reconnecting the main load if
desired.
The steps taken for parallel cells can also encompass tracking the
power of the stack and determining the slope of the power versus
current. Again, if the slope is zero or negative (negative dP/dI
region) then the load is disconnected temporarily. The main load
gets reconnected when the slope goes positive.
Accordingly, there has been provided a fuel cell system having a
protection circuit and technique to prevent operation of the system
in the negative dP/dl region. By monitoring a fuel cell parameter,
comparing the fuel cell parameter to a preset condition, and
disconnecting and reconnecting the main load depending on the fuel
cell parameter in relation to the preset condition, an effective
fuel cell protection means has been provided. The fuel cell
protection means of the present invention is particularly
beneficial to hybrid, microfuel cell systems.
In the foregoing specification, the invention has been described
with reference to specific embodiments. However, one of ordinary
skill in the art appreciates that various modifications and changes
can be made without departing from the scope of the present
invention as set forth in the claims below. Accordingly, the
specification and figures are to be regarded in an illustrative
rather than a restrictive sense, and all such modifications are
intended to be included within the scope of present invention.
Benefits, other advantages, and solutions to problems have been
described above with regard to specific embodiments. However, the
benefits, advantages, solutions to problems, and any element(s)
that may cause any benefit, advantage, or solution to occur or
become more pronounced are not to be construed as a critical,
required, or essential features or elements of any or all the
claims. As used herein, the terms "comprises,""comprising," or any
other variation thereof, are intended to cover a non-exclusive
inclusion, such that a process, method, article, or apparatus that
comprises a list of elements does not include only those elements
but may include other elements not expressly listed or inherent to
such process, method, article, or apparatus.
* * * * *